It has been found that some uranium ores are not amenable to alkaline or neutral carbonate leaching. These refractory ores are readily leached using dilute sulfuric acid (about 0.1-1% by weight) and oxygen under pressure. From the standpoint of cost-effectiveness, sulfuric acid is the most desirable leaching chemical for use in the in situ leaching of uranium ores. HCl is more costly and interferes with the subsequent ion exchange processes used for the recovery of uranium values from the leachate solution that is produced from the formation. HNO.sub.3 is very expensive, and its potential for water pollution makes it unsuitable as a leaching chemical. Other acids such as acetic acid are much more expensive than sulfuric acid and interfere with the recovery of uranium values by ion exchange.
Unfortunately, sulfuric acid has a long recognized disadvantage as a leaching chemical for ores which contain calcite (or CaCO.sub.3), namely, the potential plugging of the formation by the precipitation of CaSO.sub.4. Various methods have been devised in the past to circumvent this problem of CaSO.sub.4 plugging and the resulting loss in the formation permeability, but they have not been particularly successful. The importance of this problem is underscored by the fact that uranium-bearing ores are most often found in sandstone formations. There is always some calcium carbonate present in sandstone. For example, the CaCO.sub.3 content of uranium ores at Crownpoint, New Mexico, which are refractory to alkaline and neutral carbonate leaching, is in the range of from 0.6% to 6.20%.
The problem of calcite control in an in situ leaching operation is addressed in U.S. Pat. No. 4,103,963, which describes an in situ leaching operation which is representative of the prior art to this invention. The mechanics of the invention may generally follow the disclosure of that patent. Thus, it is believed to be readily apparent to persons skilled in this art upon reading the disclosure of U.S. Pat. No. 4,103,963 in conjunction with the disclosure herein how to practice the present invention.